On back shelf of car in direct sun, all 4 windows open.
It only took a few minutes to reach over 50° Celcius

The RSPCA advice is: if you see a dog in a car on a warm day, please call the police.

Under the Animal Welfare Act everyone have a legal duty to care for their animals and if someone puts their animal at risk, they should face prosecution. Obviously they also have to live with the fact that their action resulted in terrible, unimaginable suffering.

If the car is in a supermarket car park or at a public event, ask for a message to be broadcast, asking the owner to return to their car immediately.
There has been much debate online as to what to do if no one comes in time and the clock is ticking. Consensus is to try to take photos/video of the scene, using your phone. If you decide things are serious and you need to break in to release the dog try to make sure you record what you are doing and to protect yourself from accusations of criminal damage. Certainly advise the police of your intention to do this, so they are aware.

Heatstroke early warning signs

Heavy panting

Profuse salivation

A rapid pulse

Very red gums/tongue

Lethargy

Lack of coordination

Reluctance or inability to rise after collapsing

Vomiting

Diarrhoea

Loss of consciousness

Heatstroke first aid

If a dog shows any symptoms of heatstroke, move him/her to a shaded, cool area and ring a vet for advice immediately. Heatstroke should always be treated as an emergency.

Dogs suffering from heatstroke urgently need to have their body temperature gradually lowered: